The invention relates to a laser-doppler-anemometer for measuring the velocities of moving test specimens, with deflection optics for producing at least two partial beams, collecting optics for focussing at least one of the partial beams on the test specimen, and a detector for the scattered light radiated from the moving test specimen.
Such laser-doppler-anemometers are known and their measuring principles are described in detail in "Principles and Practice of Laser-Doppler-Anemometry" by Durst, Melling, Whitelaw, 2nd edition, Academic Press 1981.
In the known laser-doppler-anemometer the beam from a gas laser is split into two partial beams. Using the cross-beam method, the two partial beams are directed at a flowing medium at different angles, where they produce a virtual interference pattern in the measurement volume. The beams passing directly through the measurement volume are cut off, while the beams scattered in another direction from the measurement volume are detected by means of a photomultiplier. The frequency of the scattered light is dependent on the velocity of the moving test specimen, so that a measurement of the flow velocity is possible.
In the reference beam method only one partial beam passes through the measurement volume. The scattered partial beam is made to interfere with a reference beam.
The measurement method of anemometry has the advantage that for practical purposes it can be used on the moving test specimen, which may, for example, take the form of a flowing medium or a moving surface, without producing any reaction at all. It is necessary only that the moving test specimen contain light-scattering particles. These can be added to a gas or liquid flow even before the measurement is made.
The disadvantages of previous anemometers resides primarily in the fact that the necessary inclusion therein of gas lasers make them very bulky and heavy. Moreover, with fixed deflection optics, the laser has to be moved in order to direct the partial beams into the measurement volume, so that an expensive device for moving the gas laser is required. The same applies to the receiving optics.